In root canal therapy, the interior of a root is removed prior to filling the root with replacement material. If the canal is not completely emptied of the root material prior to filling with the replacement material, leftover root material can retard healing and even act as a focus for infection. For this reason, all of the natural interior material in the root is removed before filling.
Both mechanical and chemical means are used to eliminate such germs in the root canal in an effort to prevent further infection. Traditionally this has been done using a mechanical means such as a reamer or a file. Also, the root canal should be fully closed almost all the way to its apical foramen so as to completely prevent a recurring infection. Accordingly, it is essential to accurately determine the position of the end of the root canal, known also as the apical foramen. In order to measure the length of the root canal, different methods have heretofore been used. In general it is conventional to insert a probe into the excavated root canal and to determine how far the tip of the probe is from the apex of the root.
This method may comprise, e.g. determining by hand when a probe such as a file or reamer is in contact with the apical foramen, taking an X-ray with said probe inserted in the root canal, or also electrically detecting the apical foramen by means of impedance variations. Once the apex of the tooth has been determined, a mechanical cutter such as a hand-operated reamer or file, or an engine reamer or ultrasonic cutter, is then frequently used to enlarge and clean the root canal.
It is, however, difficult even for a skilled dentist to efficiently perform such a series of operations, which are time-consuming and may cause patients to suffer severe pain.
Different prior art systems have been devised to accelerate this method. In one prior art system, which is based on an impedance measurement, an electrical measuring means has an alternating power source with a frequency of 200 Hz. When the reamer reaches the apical foramen, the value of a voltage is measured. This clinically pre-determined value is then used as a reference value indicating when the reamer has reached the apical foramen for future measurements. In another system, developed for detecting any given position in the root canal by means of impedance differential, voltages are used that have two different fixed frequencies, for example, 1 kHz and 5 kHz. Waveforms of those frequencies are superposed to provide a composite or voltage waveform. In this system (see U.S. Pat. No. 5,080,586, the disclosure of which is incorporated herein by reference), the apex is located by electrification of the probe with applied pulses at two different fixed frequencies and measuring a current flowing between the probe and an electrode at the gum of the patient. The measured current is filtered to provide only the fundamental frequency to an apex location determination circuit. The changes in the two filtered signals is compared as the probe moves along the root canal and resulting changes in the current values are used to determine the apex location. This current is measured by measuring the voltage on a resistor situated between the gum electrode and ground.
The aforementioned prior art systems, however suffer from an insufficient signal sensitivity especially when the probe is in close proximity of the tooth apex. To overcome these deficiencies, the present invention provides an improved system with an improved signal to noise ratio and, thus, an improved accuracy as compared to prior art methods. Thus, a more accurate determination of the apex in a shorter time span becomes possible.